A contest between two individuals, no more, no less (er, obviously). Despite what Mr. Anonymous would have you think, it is not necessarily a battle to the death. A chess match could be considered a duel, as could a swordfight, or a naked mudwrestling contest (between women, of course. Naked men mudwrestling is a fucking crime against nature).

"I challenge you to a duel! Pick up your pugel stick! GLADIATORS, RRRRRREADY?"